Serotype Switching Research Articles

The 44-kb linear plasmid molecule in the relapsing fever agent Borrelia duttonii strain Ly serve as a preservation of vmp genes.

Borrelia duttonii strain Ly, a causative agent of relapsing fever, contains a linear one megabase chromosome and 12 linear plasmid molecules. Here we report that the sequence of the 44-kb linear plasmid of strain Ly is found to contain variable major protein (vmp) genes for antigenic variation of relapsing fever borreliae. The determined sequence is of 44,010 bp except for both ends of the molecule. Of 39 open reading frames (ORFs) found in the sequence, 21 ORFs (named vmpA to U) showed moderate similarities with vmp genes for Borrelia hermsii. However, most of the vmp homologues are apparently nonfunctional because of their frameshifts within the sequence and/or absence of promoter and ribosome-binding signals upstream of their genes. RT-PCR experiments using the specific primer for each vmp gene revealed that vmpE, one of the vmp genes, was expressed at the location of the 44-kb plasmid molecule. The result suggests that the plasmid molecule may play a role in the preservation of the serotype switching of vmp genes in a mammalian host.

Recombinant adenovirus serotype switching for analysis of efficiency and route of intestinalcell entry

Recombinant adenovirus serotype switching for analysis of efficiency and route of intestinalcell entry

Population biology of Streptococcus pneumoniae isolated from oropharyngeal carriage and invasive disease.

The population structure of Streptococcus pneumoniae in a sample of 134 carried antibiotic-susceptible isolates, and 53 resistant and susceptible invasive isolates, was examined using a DNA-based version of multilocus enzyme electrophoresis: multilocus restriction typing (MLRT). This involved RFLP analysis of PCR products generated from nine loci of housekeeping genes located around the pneumococcal chromosome. The combination of alleles at each of the nine loci gave an allelic profile or restriction type (RT). All carried (throat or nasopharyngeal) isolates from children or adults in Oxford and Manchester, UK, and from an HIV-seropositive cohort in Nairobi, Kenya, showed an epidemic population structure. Twelve carried clonal groups, each with different serotypes, were identified at both locations within the UK. Almost all of the carried clones examined (16/17) were found to possess identical RTs or sequence types (STs) to invasive isolates, indicating that frequently carried clones are also associated with cases of invasive disease. As expected from previous studies, the population of 53 invasive, mainly penicillin-resistant, isolates was also found to be at linkage equilibrium. Serotype switching was identified among 14% of RTs that possessed two or more members, or 5.7% of individual isolates within these RTs. In support of a population structure in which there is frequent recombination, there is also clear evidence that the trpA/B locus within pneumococci has evolved by horizontal gene transfer. A non-serotypable isolate from an HIV-seropositive patient in Kenya was clearly genetically distinct from other strains studied, with unique alleles at eight out of nine loci examined. However, it was initially identified as a pneumococcus by a 16S RNA gene probe (Gen-Probe), optochin susceptibility and the presence of pneumolysin and autolysin.

Microbiological aspects

By the 1930s several studies had shown that Streptococcus pneumoniae was an important pathogen in Nairobi (Kenya) and various risk factors for infection were recognized, including seasonally cold conditions, overcrowding and recent arrival in the city. Research into pneumococcal disease declined with the arrival of penicillin but recently interest has been rekindled by recogńition of the pneumococcus as a human immunodeficiency virus (HIV)-associated pathogen and by the emergence of antibiotic resistance. The pneumococcus and its association with HIV were studied during the course of the Wellcome Trust/Kenya Medical Research Institute HIV Programme in Nairobi (1988–1993). There were generally high rates of pneumococcal disease. The pneumococcus (with tuberculosis and salmonellosis) was a major HTV-related pathogen. One study showed HIV seropositivity to confer a relative risk of 17·8 for pneumococcal infection. Recurrent infection accounted for a large proportion (25%) of disease episodes in a longitudinally studied cohort of HIV patients. There were higher pneumococcal carriage rates in HIV-positive than in HIV-negative patients (28% vs. 16%, P = 0.003). High rates of resistance were found to penicillin (25%). Molecular characterization of penicillin-resistant strains identified 11 separate clones, showing great genetic diversity in a small sample of isolates, and there was evidence of horizontal spread of penicillin-binding protein genes between separate lineages. Molecular characterization of isolates from patients with recurrent disease suggested that both relapse and reinfection might occur. There was molecular evidence of transfer of capsular genes between clones (serotype switching). The overall spectrum of serotypes resembled those reported elsewhere, most serotypes being included in the 23-valent vaccine. Higher numbered serotypes were associated with respiratory tract source and antibiotic resistance. Various methods were used to show 82% concordance between pernasal and blood isolates in pneumonia cases. HIV-seropositive patients were more prone to infection with penicillin- and tetracycline-resistant organisms than seronegative patients (penicillin, 27% vs. 7%; tetracycline, 40% vs. 17%), a difference reflected by different serotype profiles in the 2 groups. These studies highlight the importance of the pneumococcus as an HIV-related pathogen in one part of Africa. The high rates of antibiotic resistance are a cause of concern. There should be continued monitoring of resistance patterns, and assessments of the significance of penumococcal disease made elsewhere in Africa are to be encouraged.

Colonization with penicillin-nonsusceptible Streptococcus pneumoniae in urban and rural child-care centers.

To determine the prevalence of penicillin-nonsusceptible Streptococcus pneumoniae (NS-SP) at 12 child-care centers (CCC) in urban and rural Nebraska and to evaluate the genetic diversity of pneumococcal strains present in the CCC environment. Nasopharyngeal cultures for S. pneumoniae were obtained from children 2 to 24 months old. Capsular serotyping, pulsed field gel electrophoresis (PFGE) and microbroth dilution MICs were performed for all S. pneumoniae. Antibiotic exposure was also evaluated as a potential risk factor for colonization with NS-SP. Nasopharyngeal colonization with S. pneumoniae was present in 121 (56%) of 215 children. The MICs of penicillin were 0.12 to 1.0 microgram/ml for 57 (47%) and > 1.0 microgram/ml for 10 (8%) isolates. Clindamycin MICs of > 0.5 microgram/ml were found in 6 isolates (5%). MICs of ceftriaxone were 0.5 microgram/ml in 28% of S. pneumoniae and 1.0 microgram/ml in 7%. PFGE and capsular serotyping demonstrated multiple strains that were penicillin-nonsusceptible in both the urban and rural CCC. PFGE and capsular serotype defined shared strains within each CCC, but some PFGE "types" could be found in multiple serotypes. Antibiotic exposure during the 2 months before nasopharyngeal culture was not a statistically significant risk factor for nasopharyngeal colonization with NS-SP. NS-SP are highly prevalent in urban and rural Nebraska. PFGE similarities between serotypes may reflect "serotype switching" but may also reflect genetic similarity between S. pneumoniae strains.

Serotype switching in a partially deleted RHD gene.

We have studied the RH genes in donors with the RhD-negative haplotype dCes. In contrast to the usual arrangement of genes in RhD-negative individuals, where the lack of antigen expression is due to deletion of the entire RHD gene, we find that the dCes haplotype includes an RHD gene with an internal deletion. Moreover, there appear to be no 5' sequences characteristic of RHC suggesting that the RhC antigen may be encoded by a truncated RHD or a recombinant RHD/CE gene in these dCes/dce genomes.

Serotype Switching in a Partially Deleted RHD Gene

We have studied the RH genes in donors with the RhD-negative haplotype dCe^s. In contrast to the usual arrangement of genes in RhD-negative individuals, where the lack of antigen expression is due to deletion of the entire RHD gene, we find that the dCe^s haplotype includes an RHD gene with an internal deletion. Moreover, there appear to be no 5' sequences characteristic of RHC suggesting that the RhC antigen may be encoded by a truncated RHD or a recombinant RHD/CE gene in these dCeVdce genomes.

Antigenic variation of surface proteins of Borrelia species.

During relapsing fever, the etiologic spirochetes employ antigenic variation to avoid early immune clearance. In Borrelia hermsii, an agent of tick-borne relapsing fever, a switch in serotype is associated with a change in the predominant protein on the surface of the cells. The variable major proteins differ from one another in primary structure along their lengths. Genes encoding the different variable major proteins can be found in both "silent" and "active" environments. At the active site the gene is expressed; at the silent site the gene appears to be in storage. In variant cells, which express the new serotype-specific surface protein in a population, a gene from a silent site has replaced the previously sitting gene at the active site through transposition.